Armour was born in Stockbridge, New York to Danforth Armour and Juliana Ann Brooks; he was mostly of Scottish and English descent, with his surname originating in Scotland.[1] He was educated at Cazenovia Academy in New York before he dropped out and went to work on the family farm. In 1852 he walked across the country to mine the gold fields of California, and earned US$8,000 by the time he was 24. He used those funds to set up his first meat market in Placerville, California.

He moved from California to Milwaukee, Wisconsin, with a sizeable fortune and started a wholesale grocery business. With his brother, Herman, he entered the grain business and built several meat packing plants in the Menomonee River Valley. Together they formed the Armour and Company in 1867, which soon became the world's largest food processing and chemical manufacturing enterprise, headquartered in Chicago, Illinois.

In order to get his meat products to market Armour followed the lead of rival Gustavus Swift when he established the Armour Refrigerator Line in 1883. Armour's endeavor soon became the largest private refrigerator car fleet in the U.S., which by 1900 listed over 12,000 units on its roster, all built in Armour's own car plant. The General American Transportation Corporation would assume ownership of the line in 1932.

In 1893, he donated $1 million to found the Armour Institute of Technology (a privately endowed coeducational college), which merged with the Lewis Institute to become the Illinois Institute of Technology (IIT) in 1940.

His meat packing plants pioneered new principles of large-scale organization and refrigeration to the industry. Armour was one of the first to take action to reduce the tremendous waste inherent in the slaughtering of hogs and to take advantage of the resale value of what had been waste products.

The company's reputation was tarnished by the scandal of 1898-99 in which it was charged with selling tainted beef. This event provided fodder for the muckraking novel by Upton Sinclair entitled The Jungle, which was published in February 1906 and became a bestseller.

Armour died on January 6, 1901 of pneumonia at his Chicago home. The town of Armour, South Dakota was named for him in 1885. A street in the Milwaukee suburb of Cudahy, Wisconsin (founded by meat packing magnate Patrick Cudahy) also bears his name.

Philip married Malvina Belle Ogden Ogden. (Malvina Belle Ogden Ogden was born in 1842 and died in 1927.)